The sensitivity of on-chip integrated photonic devices to external effects acting on the chip or on the local environment of the photonic device is a major issue, in particular for integrated photonic devices with an intentional wavelength-dependent transmission or filter characteristic. The dependence of device characteristics on temperature, geometrical variations (e.g. related to manufacturing tolerances) and/or ionizing radiation, is undesirable for such integrated photonic devices.
For example, the problem of temperature sensitivity is in particular pronounced for photonic devices made in a material having a high dependence of refractive index on temperature (i.e. a high thermo-optic coefficient). Therefore, temperature dependence is one of the fundamental limitations of silicon photonic devices, because of the high thermo-optic coefficient of silicon (1.86×10−4 K−1). It is a challenge to reduce the temperature sensitivity of such photonic devices, in particular photonic devices with a wavelength-dependent transmission or filter characteristic, especially in combination with a low propagation loss and small footprint.
The temperature dependence of integrated silicon photonic devices can be reduced by providing an overlay as a top cladding layer, wherein the overlay material (e.g. a polymer) has a thermo-optic coefficient opposite to the thermo-optic coefficient of silicon. However, the use of such an overlay may lead to aging problems and thermal hysteresis. Moreover, a polymer overlay cannot withstand high-temperature treatments typically used in back-end metallization in CMOS processes. Therefore the use of such a polymer overlay is not CMOS compatible.
Another solution is the use of local heaters to dynamically stabilize the photonic device. However, this requires active temperature monitoring which is space consuming and leads to high power consumption.
In US 2011/0102804 a Mach-Zehnder interferometer (MZI) is described that can be made athermal (temperature independent) by using different waveguide arm widths and selecting proper arm lengths such that the temperature sensitivity of one arm cancels that of the other arm. This is based on the different response of the waveguides to changes in temperature because the fraction of light that is confined in the silicon waveguide core is different for both waveguides.
Similarly, integrated silicon photonic devices are very sensitive to variations in line-width and thickness (e.g. induced by the fabrication process) and to ionizing radiation.